Electronic synchronizing system



April 22,y 1952 F. A. HESTER ELECTRONIC SYNCHRONIZING SYSTEM Filed July 28, 1949 m. 9 :NV

A TTORNE Y April 22, 1952 F. A. HEsTER ELECTRONIC SYNCHRONIZING SYSTEM 4 Sheets--Sheel 2 Filed July 28, 1949 y my m E IW L F,

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April 22, 1952 F. A. HEsTER 2,593,450

ELECTRONIC SYNCHRONIZIJIG SYSTEM Filed July 28, 1949 4 Sheets-Sheet 3 INVENTOR. FRANK A. HESTER ATTORNEY April 22, 1952 F. A. HEsTER 2,593,450

ELECTRONIC SYNCI-IRONIZING SYSTEM Filed July 28, 1949 4: Sheets-Sheet 4 c: u G o J Z L- l j O KE :I N I" a :s u. o

.o f Q -I- :2 8) I 2 1"' I I I I Q I m 5 J I I I I I @I I I'n 1 9 s h. I I I I I I INVENTOR. FRANK A. HESTER BY ATTORNEY Patented Apr. 22, 1952 ELECTRONIC SYNCHRONIZING SYSTEM Frank A. Hester, New York, N. Y., assignor to Faximile, Inc., New York, N. Y., a corporation of Delaware Application July 28, 1949, Serial No. 107,161

9 Claims. (Cl. 319-341) This invention relates to systems for synchronizing the operation of one equipment with the operation of another equipment, and more particularly to synchronizing systems of the electronic type. While the present invention relates to a system for synchronizing the speed of a mtor with received synchronizing signals, it will be understood that various subcombinations of the invention are adapted for use in many ways apparent to those skilled in the art.

, While not limited thereto, the present invention is particularly useful when applied to the facsimile art wherein it is common practice to scan copy line by line and to utilize a space between the signals representing each line for the transmission of a synchronizing signal. At the receiving end, the recording electrode may be driven by a motor which must be synchronized with the transmitted signals to reproduce the copy line by line. synchronizing systems have often included brushes and a commutator driven by the motor. While these systems of the prior art have been found to operate very successfully, they suffer from the disadvantage that the commutators and brushes require frequent cleaning and other attentions. It is therefore a general object of this invention to provide a synchronizing system which is reliable and troublefree by reason of its not requiring the use of commutators and brushes or other mechanical switching devices.

Another objection to synchronizing systems of the prior art is that they display inertia effects, i. e., in correcting a lead or lag, they tend to overcompensate and hunt This failing is especially objectionable in facsimile equipment because a permanent record is made which continues to give testimony to the imperfectness of the equipment. It is therefore another object of this invention to provide a synchronizing system which isfree from inertia effects; i. e., one which does not compensate by means of progressively smaller overcompensations.

A further object is to provide a synchronizing system which provides initial synchronization in a very short period of time and which maintains perfect synchronization thereafter.

A further object is to provide a counter-gate device receptive to a square wave or to periodic pulses and operative to count the pulses and allow every nth pulse to go through and appear on the output terminals without change in its shape or duration.

A further object is to provide an apparatus adapted to deliver to a motor an alternating driving current which is in exact synchronism with the relatively weak output of a vacuum tube oscillator applied to the apparatus, the apparatus being much simpler and less expensive than a conventional vacuum tube power amplier.

Pursuant to these and other objects which will be apparent to those skilled in the art, the present invention involves a, motor driven by an A.-C. power supply controlled in frequency by a variable-frequency oscillator receptive to a frequency-determining D.C. voltage stored on a capacitor. A portion of the oscillator energy is applied to an overdriven amplifier to generate a corresponding square wave, which, in turn, is applied to a counter-gate having an output of one half-cycle square pulse per n cycles of the square wave generator. This local pulse is differentiated and the leading spike applied to a circuit for discharging the frequency-determining storage capacitor of the oscillator to a base value. The local pulse and a received synchronizing pulse of comparable width are applied to a thyratron circuit for recharging the frequencydetermining storage capacitor to a value which is a function of the duration of time that the pulses overlap. In the normal synchronized condition of operation the pulses will overlap by a certain amount, as, for example, per cent. The mode of operation is'such that the frequency of the oscillator is constant during the time between pulses. A synchronizing pulse causes the capacitor to be discharged to a base value and then quickly recharged to a value which determines the frequency of the oscillator until the next pulse arrives.

For a better understanding of the circuits of the present invention and the manner in which they cooperate in the system, reference is had to the following description taken in conjunction with the appended drawings, wherein:

Figure 1 is a facsimile receiving equipment having a synchronizing system constructed in accordance With the teaching of this invention.

Figure 2 is a representation of waveforms which appear at various designated points in the circuit of Figure 1 and which will be referred to in describing the operation thereof.

Figure 3 is an alternative form of counter-gate which may be used in place of that shown in Figure 1.

Figure 4 is another alternative form of countergate.

Figure 5 is a form of power amplifier which may be used between the variable-frequency oscillator and motor of Figure 1.

Referring now in greater detail to the draw-` ings, facsimile receiver I receives facsimile signals from an antenna II and after conventional translations applies them through a recorder amplifier I2 to a linear recording electrode I3 and a helical recording electrode I4. The electrode I4 is rotated by motor I5, and electrolytic recording paper I5 passing between the recording electrodes is marked in accordance with the copy scanned at a facsimile transmitting station. The speed of rotation of helical electrode I4 is determined by the frequency of A.C. power supplied to synchronous motor I5 from variable-frequency oscillator I1 through power amplifier I8. The oscillator I1 includes terminals I9 across which a D.C. voltage may be applied to determine the frequency oi the oscillator.' A storage capacitor is employed across the terminals I3 as a voltage or energy storage device which is periodically acted upon by synchronizing signals. The variable-frequency oscillator I1 may be of the type described in a copending application of Frank A. Hester, Serial No. 689,192, led August 10, 1946.

A portion of the energy in the output circuit of oscillator I1is conveyed by wire 25 to the input of an overdriven amplifier 26 wherein the sine wave A (of Figure 2) is transformed into a corresponding square wave B of the same frequency. The square wave is applied through coupling capacitor^21 to a voltage divider 23 forming a part of a counter-gate circuit generally designated 29.

Counter-gate 29 includes al gaseous discharge tube 30, as for example a thyratron, having a heated cathode 3l, a grid or ring electrode 32, and an anode orv plate 33. The plate 33 is connected to the tcp of voltage divider 28 and the cathode is connected to ground through a load resistor 34. connected through resistor 36, rectifier 31 and rectifier 38l to ground. Rectiers 31A and 38 may be thermionic diodes or'germanium crystals. A grid biasing capacitor' 4D is connected from cathode 3| to point 4I between the rectiers, and, in series with resistor 42, to the grid 32. A capacitor 43 connected between a tapk onv voltagel dividerV 28 and grid 32 acts together with resistor' tion of the pulse and this leaves a charge on grid f biasing capacitor which prevents the tube 30 from conducting when the next positive pulse 45 is applied to the tube. This next positive pulse, however, partially discharges capacitor 40 by reason of current flowing from tap 35 through resistor 36 and rectifier 31 to the negative side of the capacitor, the amount of discharge being determined by the voltage on capacitor 40, the time duration of the pulse and the values of the R-C constants in the circuit of resistor 36 and capacitor 40. Successive pulses applied to tube 30 progressively discharge capacitor 40 until an nth pulse 46, where n mayA for example be 10, causes the tube to conduct and recharge capacitor 40. The cycle of operation is then repeated. It is to be noted that the tube 30 always begins to conduct at the beginning of the .nth pulse be'- cause a pip is applied to grid 32 from differentiator 42, 43r at the beginning of every pulse. As the charge on grid-biasingA capacitorl 4U is' de- A tap 35 on voltage divider 28 is 4. creased, the pip applied to grid 32 insures that the tube will iire only at the beginning of a pulse and never between pulses or during a pulse. The circuit has therefore been termed a countergate because it counts the pulses and allows every nth pulse to pass through and appear across the load resistor 34. The circuit has been found to be positive and consistent in operation when n is in the order of l0 and less.

Since the counter-gate output waveform, illustrated as C in Figure 2, includes disturbances 41, it is applied by connection 48 to an inverterlimiter 5S, the output of which is applied through a coupling capacitor 5i to a junction point 52. The voltage appearing at junction point 52 across resistor 53 (D of Figure 2) is applied through wire 54 to a circuit for discharging capacitor 20 which subsequently will be described, and through wire 55 to a circuit for charging capacitor 20.

The charging circuit includes a gaseousv discharge tube cc having a bias battery 6| connected in series with resistors 62 and 63 between the cathode 64 (or ground) and grid 65. The' plate 63 is connected through a charging resistor 51 to the capacitor 2B. The cathode 64 is connccted to wire 55 and the grid B5 is connected through resistor E3, coupling capacitor 68 and.

wire B9 to the signal output of facsimile receiver I0.

In operation, the received facsimile signal ori wire 69 is as represented by G of Figure 2. The

fluctuations 1G correspond to one point-by-point line'of the copy transmitted by a facsimile transmitter. The beginning of the next adjacent line is designated 1I. Transmitter synchronizing pulses 12 are received between the signals corresponding to lines of copy. Tube Si) is biased byv such an amount that it will not conduct unless there is simultaneously present a negative pulse on cathode'Sd from junction point 52 and a posi'- tive synchronizing pulse on grid 65 from receiver I0. During the time that the pulses coincide in time, the tube conducts, drawing a current from capacitor 23 through charging resistor' 61.

The voltage on the ungrounded side of capacitor 20 is thus lowered from a base value 1'3 to a more negative value 14 (H of Figure 2'). The value ofthe resulting charge is determined'. by circuit constants and the charging duration iti-t2, i. e., the time during which the pulses 44 and 12 coincide. The charge on the capacitor 2) determines the frequency of the variable frequencyoscillator I1. remain'constant for all practical purposes during the interval between pulses or the time it takes to scan one line with present and contemplated equipment. By the selection of charging circuit elements to provide the proper time constant, the charging curve 15 may be made practically linear so that the frequency-determining voltage- 14 isa linear function of the overlap duration ti-tz of pulses 44 and 12,t The frequency of oscillator I1 may therefore be adjusted to the correct value in one cycle, there being no hunt-- ing or inertia effects. .l

Having described the means whereby capacitor 20 is charged, the means for discharging the capacitor will now be considered. Waveform D at junction point 52 is applied to inverter-limiter from which it emerges as waveform E. Waveform E is applied to diiferentiator SI consisting of capacitor 82 and resistor 83. The output of differentiator 8 I, having waveform F, is applied to Igrid 84.v of vacuum tube S51 The plate 86 oftubeY The charge and frequency 85 is connected to positive B voltage; and the cathode 81 is connected through a variable resistor 88 to point 89 from which there are two paths to ground, 4one being through discharging resistor 95 and capacitor 20, the other being through rectifier 9I. The polarity of rectifier BI is such as to prevent discharge therethrough of the charge on capacitor 20. A grid bias source 94 is connected to resistor 83 to normally maintain tube 85 below cutoif.

In the operation of the capacitor discharging circuit, tube 85 conducts when a positive spike 92 (waveform F) is applied to grid 84. Current then iiows from the B source, through tube 85 and resistor 83 to point 89 Where the current divides, part going through rectifier BI to ground and part going through discharge resistor 9i! to reduce the negative voltage on the ungrounded side of capacitor 20. The discharging of capacitor 20 may be viewed in another Way by considering that the current flowing through resistor 83 to ground via rectifier 9| places point 89 approximately at ground potential. Therefore, capacitor 20 is discharged to ground through resistor 99. The capacitor 20 is discharged by an amount which is a function of the charge on the capacitor, the width of spike 92 and the R-C time constant. Negative spike 93 has no eiect on tube 85 since the tube is cut oilc at all times except when a positive spike 92 is applied.

The frequency of oscillator I'I is determined within a range of values by the charge on capacitor 20, the desired synchronized frequency being in the central portion of the range. rEhe oscillator I1 of the system of Figure l may be designed so that when it is initially put into operation, the frequency is at the low end of its range of values. Since the pulses 44 and the synchronizing pulses 'I2 initially have different repetition rates, one pulse 44 and one pulse 'I2 will overlap within a small `fraction of a second after the equipment is started. When this occurs, the capacitor 2t is charged up to a value which is a function of the amount of overlap of the pulses. The charge put on the capacitor 20 makes the frequency of the oscillator close enough to the synchronizing frequency so that the next pair of pulses 44 and 'I2 also overlap. As previously described, the capacitor 2i) is discharged at time to corresponding With the leading edge of pulse 44 and is recharged durin-g the overlap period ti-tz to a frequency-determining value I4 until the next pair of pulses appear. As is apparent from H of Figure 2, the frequency of Voscillator I'I is constant during the period between pulses and corrections in frequency are made only during the occurrence of the pulses. The frequency correction made is a function of the overlap of the pulses and is substantially independent of the previous frequency of the oscillator.

Reference now will be made to Figures 3 and 4 for descriptions of alternative forms of the counter-gate 29 of Figure 1. The counter-gate 29 cf Figure 3 differs from counter-gate 29 in that the circuit for discharging capacitor 4B is connected by wire IUD to the input side of the next preceding stage IGI of overdriven amplifier 26, instead of to point 35 on voltage divider 28. By this arrangement, capacitor 40 is step-by-step discharged by positive pulses during the occurrence of corresponding negative pulses on voltage divider 28 and grid 32'; and the ratio of the voltage on wire |09 to the maximum voltage on capacitor 40 may be made larger than is practical in the circuit of Figure 1 so that the capacitor 6 40 is discharged by more nearly equal amounts. As a result, the counter-gate of Figure 3 is more positive in operation, i. e., there is less possibility of tube 3u firing on the occurrence of a pulse other than the predetermined nth positive pulse.

The counter-gate of Figure f1.. is also positive and consistent in operation by reason of its nearly equal capacitor discharge steps. Like the form shown in Figure 3, it counts the negative pulses and gates the nth positive pulse. A gaseous tube Ile has its plate ii i connected to input terminal I I2 and through a capacitor 23 to a voltage divider IM. The grid H5 of tube II is connected tc tap i iii on voltage divider I I4. The cathode II'i' is connected through output resistor IIB to ground, through discharging resister II9 and rectifier i253 to input terminal IIZ, to output terminal I2 i, and through bias-storing capacitor 122 te point |23 which is connected to the loWer end of voltage divider IM, rectifier I24 and capacitor IE5.

When a pcsitive pulse is initially applied from terminal H2 through capacitor H3 and part of voltage divider I it to grid H5, the tube IIB res and current passes through load resistor II8 to ground. The resulting voltage drop appearing across resistor IIS causes a charge on bias-storing capacitor I22 and then this charge distributes itself on capacitors V22 and iL in accordance with the relative values of capacitance and with the polarities indicated on the drawing. The charges on the capacitors bias the grid IIS so that subsequent positive pulses or half cycles do not render tube itil conductive. Each sub'- sequent negative pulse or half cycle passes from input terminal i I2 through rectifier I2@ and discharging resistor HQ to reduce the biasing voltage on capacitor |22. .Thus when the nth positive pulse appears at terminal H2, the bias has been reduced to a value such that the tube IID conducts and gives a corresponding output pulse on terminal IZB across lead resistor H8. The tube always starts to conduct at the beginning of the nth pulse because capacitor II3 and voltage divider H4 act as a diiierentiator so that a starting spike is applied to the grid II5 of the tube. The circuit cf Figure 4 is characterized by providing almost equal discharge steps on capacitor I22 rather than exponentially decreasing discharge steps. The tube is therefore positively fired by the nth positive pulse and not by the n-plus-one or the n-ininus-one pulse.

Reference will now be made to Figure 5 for a description of a power apparatus i8 which may be substituted for the conventional power` amplii'ier I8 between the variable frequency oscillator Il and nictor I or" Figure l. The purpose of power apparatus IS', like that cf power amplifier I8, is te generate fluctuating current in exact synchrenisrn with the low-power variable-frequency oscillator il' and of sufficient magnitude to drive the motor l5. The sine wave output oi' oscillator El is applied lthrough a coupling capacitor I to the grid IM of a vacuum tube |32. The grid i3! is connected through a bias resistorii to ground and the cathode IS is connected to ground. The plate IE5 is connected to B plus through the primary coil I3'! of transformer |38 shunted by a capacitor ISE. The circuit thus far described and generally designated I4() is a wave squaring circuit which transforms the sine wave output of oscillator il into a substantially square wave when the values of bias resistor |33 and capacitor IBS are selected in accordance with considerations well known in the art.

The secondary coil Mil of transformer |38 has its center-point grounded and its terminals IM, M2 connected through resistors |43, M4 to the grids H45, i115 of vacuum tubes |41, i133 arranged in push-pull. The cathodes m9, l@ are ground'- ed. The plates |52 are connected through the coils of electromagnets 453, E56 to B plus. Capacitors l, S shunting the coils of the electromagnets tend to further improve the square wave form of the currents therein.

A reed itil having one end pivoted at itl and a free end disposed between electromagnets it 'and 54 is connected by wire l to niotor l5. When reed it@ is acted on by electromagnet 53, it is dra-Wn to xed contact It connected to the positive side of D.C. source ld having a voltage E. When reed I6@ is acted on by ele'ctrornagnet |511, it is drawn to nxed contact 165 connected to ground. A second IIL-C. source H55 having a voltage E is connected on one Vside to rground and on the other side to D.C. source ist and the other terminal of motor l5.

In operation, the sine wave output of oscillator il is squared in circuit ill@ and applied to a pushpull amplier circuit generally designated 451 having electromagnets in the outputthereof which are alternately energized,Y one during the negative half cycle of the input voltage and the other during the positive half cycle of the input voltage. The electromagnets act on a contact-carrying reed it@ with the result that lead H52 of motor i5 is alternately connected to ground and a voltage 2E of both D.C. sources les and ISS in series. The other lead of motor l5 is permanently connected to a voltage E of D.-C, source let. A powerful alternating current in exact synchronisni with the oscillating current of oscillator ll' is thus applied to drive synchronous motor l5.

While specific forms or the invention have been described in soins detail, it will 'ce understood that this has been done by way or illustration and that the scope of the invention is to be construed by reference to the appended claims.

What is claimed is:

1. In a synchronizing systeni, the combination of a source of received periodic synchronizing pulses, a variable-frequency oscillator having frequencyedeterniining storage capacitor, a source of local pulses which are synchronous with the oscillator` for comparison with the received pulses, a discharging circuit receptive to pulses from one of said sources and operative to discharge the capacitor, and a charging circuit receptive to pulses from both of said sources and operative to charge the capacitor to a value which is a function of the overlap time of said pulses from both sources. i

2. In a synchronizing systeni, the combination of a source of received periodic synchronizing pulses, a variable-frequency oscillator having a frequency-determining storage capacitor, a source of local pulses which are synchronous with the oscillator and which have a repetition rate and a iiXed width of the saine order of magnitude as the repetition rate and width or" the received pulses, a discharging circuit receptive to pulses from one o said sources and operative to discharge the capacitor, and a charging circuit recept-ive to pulses Vfrom both of said sources and operative to charge the capacit-or to a value determined by the overlap time of said pulses from both sources.

3. In a synchronizing system, the combination of a source of received periodic synchronizing pulses, a variable-frequency oscillator having a frequency-determining energy-storage element, a source of local pulses including electronic counter-gate means operative responsively to the oscillator to generate a local puise during every nth cycle ci the oscillator, and further electronic means 'receptive to said received synchronizing pulses and said local pulses and operative responsively to a pulse from one of said sources to set the energy-storage element to a reference condition and then operative to reset the energy-storage element to a condition representative of the overlap time of 'the pulses from both of said sources, whereby the variable-frequency oscillator is continuously synchronized with the received synchronizing pulses in the ratio of n cycles per received pulse.

4. In a synchro" 'zing system, the combination of a source of received periodic synchronizing pulses, 'a variable-frequency oscillator having a frequency-determining storage capacitor, a source of local pulses including an electronic countergate operative responsively to the oscillator to generate a local pulse during every nth cycle of the'osciliator, and ele tronic means receptive to said received synchronizing pulses and said local pulses and operative responsively to a pulse from one of said sources to discharge the capacitor to a reference voltage and then operative to charge the capacitor to a volt-.ge which is a function of the overlap time or" the pulses iroin both of said sources.

5. In a synchronizing system responsive to received periodic synchronizing pulses, the `cornbination of a variable-frequency oscillator having a irequency-determining storage capacitor, an electronic counter-gate operative responsively to .ie oscillator to generate a local pulse during every nth cycle of the oscillator, means responsive to the leading edge of said loc-al pulse to discharge the frequency-determii ng capacitor, thyratron inea-ns responsive to the local pulse and the received synoliroinzincr pulse to recharge the capacitor to a voltage which a function of the overlap time of said pulses.

6. In a system for synchronizing the speed of a synchronous motor with received periodic synchronizing pulses, the combination of a variablefrequency oscillator havinT a frequency-determining storage capacito'r, a power amplifier receptive to the output of the oscillator and operative to supply power to drive the synchronous motor, a counter-gate operative esponsively to the oscillater to generate a local pulse during every nth cycle of the oscillator, inea-ns responsive to the leading edge of said local pulse to discharge the frequency-determining capacitor, and means responsive to the lcal pulse and the received synchronizing pulse'to rech-arge the capacitor to a voltage which is a function of the overlap time of said pulses.

`IA In a system for synchronizing the speed ci a synchronous :noto-r with received periodic synchronizing pulses, the combination of a variablefrequency oscillator having a frequency-determining storage capacitor, a source of direct current and a vibrator-type inverterV connected ereto, the inverter being synchronously responsive to the output or" the oscillator and operative to supply power to drive the synchronous motor,

means for discharging and immediately recharging the capacitor each time a synchronizing pulse is received.

S. The method of synchronizing an oscillator, having a frequency-determining vstorage capacitor, with a periodic received pulse in the ratio of n cycles per received pulse, comprising the steps of: generating a local pulse during every nth cycle of the oscillator, discharging the capacitor responsively to the leading edge of said local pulse, and recharging the capacitor during the overlap time of the received pulse and the local pulse.

9. A counter-gate receptive to an input signal of periodic pulses comprising a gaseous discharge tube having a cathode, a grid and a plate; a cathode load resistor; a grid-biasing storage cae pacitor in circuit between the cathode and grid so that when the capacitor is charged in excess of a predetermined amount the tube is 'cut off; means for applying the input signal to the grid; and a circuit, including unidirectional means, receptive to the input signal and connected to the capacitor to permit the capacitor to become charged upon receipt of the first pulse thereby cutting off the tube, and operative progressively to discharge the capacitor on receipt of subsequent pulses, the charge on the capacitor being reduced after receipt of a predetermined number of pulses to the point Where the following pulse applied to 'the grid causes the tube to conduct for the duration of said pulse; whereby the equivalent of every nth pulse of the signal appears across the cathode load resistor; whereby the tube conducts upon receipt of every nth pulse of the signal.

FRANK A. HESTER.

REFERENCES CITED FOREIGN PATENTS Country 'Date Great Britain May 27, 1938 Number 

